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<h1>Interactions</h1>

<h2>Pair Potentials</h2>

<p>The following pair potentials have been coded into 
<span class="pi">pi</span>:
<ul>
<li>Lennard Jones 6-12
<div class="math">
v(r) = 4ε[ (σ/r)<sup>12</sup> - (σ/r)<sup>6</sup> ]
</div>
</li>
<li>Aziz Potential.<br/>
R. A. Aziz et al., Mol. Phys. <b>77</b>, 321 (1992).
</li>
</ul>
In addition to these potentials, the Cao-Berne approximation to the 
hard-sphere pair action has been coded as an empirical pair action.
</p>

<p>Coulomb interactions are special because of their
<span div="math">1/r</span> singularities, long range tails, and
special symmetry involving the Lens vector. The action for
Coulomb problems is <a href="CoulombAction.xhtml">
discussed in its own section.</a></p>

<h2>Primative Approximation</h2>

<h2>Pair Approximation</h2>

<h2>Long-range potentials</h2>
<p>Long range potentials require periodic images and/or
Ewald sumations. Currently these are only implemented for
the <a href="CoulombAction.xhtml">Coulomb action</a>.</p>

<h2>Specifying Interactions in the Input File</h2>
<p>
To specify the interactions, use the EmpiricalAction tag,
<div class="code">
<pre>
&lt;!-- Lennard-Jones 6-12 parameters from Seitz and Blancore,
     J. Chem. Thermodynamics, <b>26</b> p. 1207, (1996). --&gt;
&lt;EmpiricalAction species1="He" species2="He"
  ngridPoints="1000" rmin="1 pm" rmax="10 nm"
  model="LJ" epsilon="10.22 K" sigma="0.25565 nm"/&gt;
</pre>
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